1. Field of the Invention
The present invention relates to data communication systems, backup servers, and communication control apparatuses that distribute continuous stream packets to multiple clients from at least one server via an IP (Internet Protocol) network.
Priority is claimed on Japanese Patent Application No. 2004-60528, filed Mar. 4, 2004, the content of which is incorporated herein by reference.
2. Description of Related Art
Multicasting is defined in the third layer of OSI (Open Systems Interconnection), to realize the service of distributing motion pictures and so on, and various protocols compatible with multicasting are implemented and used (see as an example, “Developing IP Multicast Networks Volume 1.1” by Beau Williamson from Cisco Systems (The Cisco Press Design and Implementation Series); ISBN: 1578700779). Also, various kinds of multicast communication protocols with high reliability are proposed to support multicast communication without data loss (see as an example, “The Reliable Multicast Design Space for Bulk Transfer” by M. Handley and five others, published by Internet Society 2000).
FEC (Forward Error Correction) is described in Handley above and the methods based on FEC are generally used to recover lost packets on the multicast distribution network today. In the systems providing these methods, a receiving side corrects errors in the packets according to error recovery information, attached to the packets from the sender side. The error recovery information might include information for single or multiple packets. The error recovery information can be sent at the same time when the corresponding packets are sent or the error recovery information corresponding to different packets can be sent in parallel if it is required. Also, Handley discloses various methods for data retransmission based on sending back ACK (acknowledgement) or NACK (negative-acknowledgement) from the receiver side.
FIG. 7 and FIG. 8 show one example of the multicast communication system for broadcasting providing redundant construction network. In these figures, multicast server for broadcasting 10 provides master 1 which distributes multicast packets. 3a and 3b are L2 switches, and 4a-4f are L3 switches. An L2 switch checks and decides the destination of each packet and transmits it according to the protocol of data link layer (the second layer). Switching hub is one example of the L2 switch. L3 switch checks and decides the destination of each packet and transmit it according to the protocol of network layer (the third layer). Router is one example of the L3 switch. 6 is an apartment house which includes multiple clients that are the destination of multicast packets distribution.
A join control message (for example, IGMP join), for joining the multicast network, is sent from the apartment house 6 when each one of clients requested to receive multicast packets. Based on the join control message, L3 switch 4a and so on decide communication paths for multicast packets, and master 1 sends the multicast packets to the apartment house 6.
As FIG. 7 shows a existing communication system with multicast protocol in which routers (L3 switches 4a-4f) have redundant structure to construct IP network with high reliability. However, for example, as shown in FIG. 8, it requires a few to several tens of seconds to change the multicast distribution path dynamically because of linkage trouble in L3 switch and so on. Furthermore, the data already distributed during a few to several tens of seconds, are lost.
On the other hand, communication systems based on FEC requires at least a fixed amount of redundant data to be transmitted regardless of whether or not there is a lost packet. Therefore, it is difficult to recover a large number of continuous packets. Moreover, the communication system based on retransmission algorithm, requires extra functions like ACK/NACK responding function for client terminals, and retransmission function for routers and sender terminals. Also, communication systems, based on the algorithm promising the delivery of all packets, there is a problem that delivery timing is not reliable.
To solve the above mentioned problems, an object of the invention is to provide communication systems, backup servers, and communication control apparatuses which recover lost data for few seconds because of linkage trouble and so on, and which realizes multicast communication preventing short breaks in data communication without any changes to the existing data communication terminals and relay nodes.